1. Field of the Invention
The present invention relates to color mixing prevention in multicolor image forming apparatus such as electronic photocopiers, facsimiles, etc., which have a plurality of developing units containing different color developers.
2. Description of the Prior Art
A multicolor image forming apparatus having a plurality of developing units containing different color developers has a mechanism for making alternative selection of one of these developing units, such that the trouble of mixing different color developers by simultaneous selection of two or more developing units may be foreclosed.
Thus each of a plurality of developing units containing different color developers is subjected to switching control between the state adapted for making development (which is referred to as developing state) and the state not adapted for making development (which is referred to as non-developing state) by moving each developing unit adjacent to or away from a photoreceptor, but respective developing units are interlocked so that as one developing unit is selected and moved adjacent to the photoreceptor for the purpose of changing the color of the image, then, the other is invariably distanced therefrom.
A conventional example of such multicolor forming apparatus is disclosed in U.S. Pat. No. 3,970,042, for example.
However, such conventional multicolor image forming apparatus have a problem in that while the mixing of different color developers is prevented by making alternative selection between a plurality of developing units, it is impossible to prevent a small amount of residual developer remaining on the photoreceptor from mixing with the other color developer even if cleaning after each copying has been done.
Thus normally, in a copier, etc., a cleaning unit equipped with a cleaning blade for removing the developer remaining on the photoreceptor after transfer process is arranged on the perimeter of the photoreceptor, besides the developing units for developing the latent image on the photoreceptor, so that when the copier is in its image forming state, the cleaning blade comes in contact with the photoreceptor, to scrape off the aforementioned residual developer, thus foreclosing its conveyance downstream side of the blade. Since this cleaning blade is designed to be spaced apart from the photoreceptor when it is not in its image forming state to prevent injuring the photoreceptor, due to the operation of contacting and separating (ON and OFF) of the cleaning blade, the developer sticking on this blade or one accumulated on the spot around the position where this blade has been contact with the photoreceptor sometimes deposits on the area on the photoreceptor downstream of the blade. Or the photoreceptor is sometimes shifted, while this blade is drawn apart from its surface, so that the developer accumulated near the spot where the blade has been in contact therewith is brought to the downstream side of the blade. And heretofore, when this device is brought into its image forming state, while this blade is brought in contact with the photoreceptor and the photoreceptor is rotationally driven, the developing unit which is to be brought into its developing state has instantly been moved adjacent to the photoreceptor. Then the developer adhering on an area on the photoreceptor downstream of the aforementioned cleaning blade (this is called residual developer stripe, because striped configuration is formed by the blade), without being removed by this blade, is conveyed by the rotation of the photoreceptor, as carried thereon; as a result, it comes to mix into the magnetic brush of the developer produced by the developing unit which has already moved adjacent photoreceptor, giving rise to occurrence of unexpected color mixing.
In an apparatus in which the rotation of the photoreceptor is started before the cleaning blade is brought in contact with the photoreceptor for prevention of catching of the cleaning blade therewith at the time of starting the rotation of the photoreceptor, the developer which has stuck in a stripe configuration along the part where the tip of the cleaning blade was placed, when the cleaning blade has been, separated from the photoreceptor upon ending the image forming state (i.e., residual developer stripe), will pass beneath the cleaning blade as it is and be carried to the position of the developing unit which has already been approached to the photoreceptor, resulting in color mixing.